This application claims the priority of 196 16 437.0, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a vehicle steering system having a manual steering device, particularly a manual steering wheel operated by the driver, and steered vehicle wheels operatively connected therewith, in which a desired-value generator arrangement is operated and the steered vehicle wheels are operated by a hydraulic actuator. The latter is, in turn, controlled by an electrically operated solenoid valve arrangement whose electric valve flow is dependent on a regulating segment determined by the desired value--actual value comparison of the steering angle or of a quantity correlated therewith.
In conventional steering systems of passenger cars, the manual steering wheel and the steered vehicle wheels are mechanically positively coupled with one another for a steering operation. It is also basically known, particularly from airplane construction, to control landing flaps and tail surfaces whereby the pilot operates only a desired-value generator via a manual control device. The desired-value generator is then operatively connected by way of an electronically operating regulating system with the landing flaps or the tail surfaces. This concept, which is also called "fly by wire", has become so reliable that it is even used for passenger planes.
Basically, corresponding concepts can assumedly be used and are advantageous also for vehicle steering systems. This is particularly so because then the space otherwise required for the steering column and the mechanical positive coupling between the manual steering wheel and the steered wheels can thus be used for a different purpose and no longer has to be taken into account in developing or designing the vehicle. In addition, the risk of injury for the driver is reduced in the case of an accident because of the elimination of the steering column.
DE 38 23 413 A1 discloses a vehicle steering system of the aforementioned type, particularly for unmanned transport vehicles which are used, for example, in production facilities and are controlled by inductively operating vehicle guiding systems. In order to keep the energy requirement of the known vehicle steering system low, the hydraulic system pressure is adapted to the respective requirements. As a rule, the hydraulic system pressure has a comparatively low value which is increased if a desired-value actual-value difference cannot otherwise be controlled. This vehicle steering device therefore operates with a certain sluggishness which, however, can be tolerated in view of the always slow travelling speed of the unmanned transport systems.
DD 237 639 A1 and DE 92 05 781 U1 show electrohydraulic steering systems, particularly for agricultural machines and tractors or motor vehicle rear axles. In these applications, PID controllers can be used.
It is an object of the present invention to provide a particularly advantageous embodiment of a steering system in which the steering system is usable for normal road vehicles with a driver and passenger.
This object has been achieved in accordance with the present invention by additionally controlling electric valve current as a function of a desired-value-dependent control segment. The control segment is preferably proportional to the rate of change of the desired value.
The invention is based on the general recognition of using construction elements which are as known and reliable as possible, in the present embodiment, hydraulic actuators and solenoid valves. With respect to the regulating system between the manual steering device and the steered vehicle wheels, the present invention is also based on the concept of already permitting as the result of the control segment, a rough adjustment which takes place without any delay by way of a desired-value actual-value comparison while only the precision adjustment takes place by way of a regulating segment determined by the desired-value actual-value comparison.
Because the control segment is preferably proportional to the rate of change of the desired value, particularly with rapid steering maneuvers (e.g., when a suddenly occurring obstacle has to be avoided), a particularly delay-free operation of the steered vehicle wheels is ensured.
The regulating segment preferably comprises a proportional segment as well as, above a threshold value of the regulating difference, a differential segment of the regulating difference. Because of the fact that in the event of slight regulating differences no differential segment is taken into account, a comfortable vehicle handling is ensured particularly with possible vibrations of the vehicle (e.g., wheel imbalances). In particular, it is ensured that the regulating does not increase the occurring vibrations.